Abstract: This paper mainly introduces how the advanced nuclear power unit of third generation monitors the reactor to achieve the initial criticality through the source range detector of the external nuclear instrumentation system under the condition of low neutron flux, and compares and analyzes the change of the detector's count rate in the process of achieving the initial criticality. Through the analysis, it is found that under the condition of low neutron flux, the achieving of initial criticality of the reactor can be judged by the change of reactor start-up rate (or reactor period). At the same time, using the change rule of the detector count rate at different positions relative to the neutron source can monitor the approaching of initial criticality. This initial criticality mode can be used in the case of low neutron flux, such as the reactor start-up without neutron source.